Once you reach the final portion of the game Jigsaw will give you two options, you can either carry on your pursuit for the truth by unlocking the "Truth" door or you can seek your freedom and an end from this by going through the "Freedom" door, there's an achievement for going through each but luckily there's a small exploit which allows you to get both achievements without having to play through the game twice.
After Jigsaw has given you the option between your freedom and finding out the truth, now it's important that you choose the freedom door first, unlock the freedom door and the achievement will unlock, now hit start and select to "Reload Checkpoint". Now you'll be back at the part where you have to decided between your freedom and finding out the truth, unlock the truth door and let the ending play out and once the credits have finished the achievement will unlock!
You have to unlock the doors in that order, if you unlock the "Truth" door first you won't be able to take advantage of this exploit!
* Credit to RoosterTeeth for the video